This invention relates generally to systems for the reproduction of recorded information, such as video information recorded on a disc, and, more particularly, to systems for maintaining a light beam essentially centered on an information track on such a disc.
Video and other types of information can be stored on a disc as a succession of light-reflective and non-reflective regions along a spiral information track on the surface of a disc-shaped record carrier. In reproducing the video information, a video disc player employs an optical system for directing a radiant reading beam, such as a laser beam, onto the information track, and for detecting a reflected beam indicative of the reflectivity of the disc surface scanned by the laser beam as the disc is rotated. In a disc player of the type with which the invention may be usefully employed, the intensity of the reflected beam takes the form of a high-frequency carrier signal which is frequency modulated by the video information.
In order to store a reasonably large amount of information on the disc, successive turns of the information track must be so closely spaced that even a slight degree of disc eccentricity would cause the beam to traverse a number of adjacent tracks on each revolution. Consequently, some means must be provided for applying transverse or radial corrections to the beam position, so that it substantially follows along the center of the information track.
One solution to this problem is to provide an analog servo system in which an error signal is derived, and a corresponding correction signal is applied to position the beam as closely as possible to the center of the track. Unless such a system employes a non-central null position, some means must also be provided for distinguishing between excursions to one side of the track from those to the other side, since the signal response is essentially symmetrical about the track centerline. Furthermore, analog servo control systems employ relatively complex components requiring critical circuit adjustments, and the component characteristics may vary significantly over a period of time. Accordingly, there is a definite need for a center tracking technique of simple design, which may be adjusted to provide accurate tracking with a minimal excursion from the centerline of the track. It is therefore an object of this invention to provide a digital tracking system that satisfies this need.